A wide variety of manufacturing processes and applications, such as assembly lines, require the constant availability of adhesive material for dispensing at periodic intervals in the operation. Where relatively large quantities of adhesive are required on a constant basis in a particular application, it has been found that manual preparation of the adhesive material is inefficient, uneconomical and susceptible to human error, particularly in the metering phase. In an effort to overcome such problems, automatic meter-mix-dispensing systems have been developed for the automatic metering and mixing of two or more components in predetermined ratios to form an adhesive material having the desired characteristics. These systems may be placed in close proximity to an assembly line or other location where the adhesive is to be applied for ease of application.
Conventional meter-mix-dispensing systems typically include delivery lines for each constituent material which have some form of metering mechanism to control the proportion of materials to be mixed. The delivery lines connect to a manifold or mixing chamber in which the constituent materials are at least preliminarily mixed together. In most designs, a one-way valve such as a ball check valve is disposed in each delivery line adjacent the manifold to prevent flow of one material into the delivery lines of other materials. Once premixed in the manifold, the materials are then diverted into an active or static mixer for complete mixing prior to release into a suitable dispensing device.
Meter-mix-dispensing systems of this general design also include a flushing system for removing individual materials and/or mixed adhesive from the manifold, mixer, outlet lines and dispensing device after the system is shut down for a certain time period. In producing solvent-based adhesives, a cleaning solution capable of removing such adhesives is introduced into the manifold and flows outwardly through the system elements to the dispensing device. It has been found that conventional meter-mix-dispensing systems having standard flushing systems using cleaning solutions capable of removing adhesive components, provide satisfactory results in the production of many two-component adhesives.
A great deal of research and development has been conducted in the field of solvent-less or water borne adhesives as replacements for solvent-based adhesives, and several products have been developed which are more desirable for a variety of reasons. In particular, there has been an increasingly growing shift to the use of what may be generically referred to as water-based adhesives from adhesives such as catalyzed polyvinyl acetate, phenol resorcinol and other solvent-based adhesives. Among the reasons for this shift from solvent-based adhesives is the greater flexibility in operating temperatures provided by water-based adhesives, better appearance at the glue line, faster production rates and minimization or elimination of emissions such as vaporous solvents and formaldehyde.
One preferred water-based adhesive system comprises an aqueous polymer dispersion forming a base emulsion and a polymeric isocyanate which acts as a cross-linking agent or hardener when mixed with the base emulsion. This adhesive material will be referred to herein as an emulsion polymer/isocyanate or EPI material.
While EPI material has been shown to provide many of the advantages common to other water-based adhesives, it was found that blockage, clogging and other problems occurred in conventional meter-mix-dispensing systems used to prepare EPI materials. Inspection of the elements of such systems revealed that solid deposits form on the valves, delivery lines and other elements after only a relatively short period of time. The problem was traced to the check valves in each delivery line and the properties of the materials used to form EPI material.
Prior to this invention, the primary modification to the flushing systems of such meter-mix-dispensing devices to accommodate the production of water-based adhesives, was the substitution of water for cleaning solution since such adhesives are water soluble or water dispersible. However, the problem with the check valves and subsequent clogging of the entire system with solid deposits remained.
As is well known, a polyisocyanate reacts with water to form a polyurea. In addition, other components of the base emulsion are capable of reacting with polyisocyanate to form other solid deposits derived from polyisocyanate. Experiments indicated that after a period of normal use of a conventional meter-mix-dispensing system, water in the base emulsion and from the flushing operation as well as other components of the base emulsion react with the polyisocyanate of the crosslinker to initially form solid deposits on the check valve of the isocyanate delivery line adjacent the manifold. Eventually, such deposits become large enough to impair the ability of the valve to properly seat. Water introduced into the manifold during the flushing operation is then allowed to flow upstream into the isocyanate delivery line where it reacts with the isocyanate to form solid deposits in the delivery line, metering device and other elements of that portion of the system. These deposits act to clog or block the isocyanate delivery line and restrict the flow of isocyanate into the mixing chamber. Although some variance may be permitted in the ratio of base emulsion-to-isocyanate without compromising the desired properties of the EPI material, significant obstruction of the isocyanate delivery line and/or metering device results in the production of an adhesive having greatly reduced bonding capability. None of the flushing systems known to the inventors are capable of removing or initially preventing the deposition of such solid deposits on the check valve disposed along the isocyanate delivery line.
It has therefore been an object of this invention to provide an improved flushing system for a meter-mix-dispensing device in the production of EPI materials.
It is another object herein to provide a flushing system for a meter-mix-dispensing device in which the formation of solid deposits is prevented before valves associated with the delivery lines of the isocyanate become restricted or inoperable.
It is a further object of the invention to provide a method for eliminating the formation of solid deposits derived from polyisocyanate in a meter-mix-dispensing device.